Technical Field
The present invention relates to a force sensor, especially relates to a force sensor having a solid-state bonding (SSB) spacer between a top stack and a bottom stack of the force sensor.
Description of Related Art
FIGS. 1A-1B, 2 and 3A-3B show a prior art.
FIG. 1A shows a prior art force sensor 100 in a status before being depressed.
FIG. 1A shows a force sensor 100 which comprises a top stack 10, a pressure sensitive adhesive (PSA) spacer 14, and a bottom stack 10B. The PSA spacer 14 is sandwiched between the top stack 10 and the bottom stack 10B. The PSA spacer 14 maintains a fixed space between the top stack 10 and the bottom stack 10B. The PSA spacer 14 has a height G11 and a width W11 in a section view before being depressed.
FIG. 1B shows the prior art force sensor 100 in a status after being depressed.
FIG. 1B shows while the force sensor 100 is depressed, PSA spacer 14 changes its dimension to a height G12 and a width W12 in a section view. The height G12 is smaller than the original height G11 and the width W12 is wider than the original width W11. This is because the PSA is viscoelastic which is flowable and sticky. When the force sensor 100 is depressed, the PSA spacer 14 is influenced and deformed downward slowly. The deformation of the PSA spacer 14 influences the deformation of the piezo material 13 and causes depression signal stabilization delay. Similarly, while the force sensor 100 is released, the restoration of the PSA spacer 14 affects the restoration of the piezo material 13 and causes restoration signal stabilization delay.
The disadvantage for the prior art is actuation signal stabilization delay when the force sensor 100 is depressed with a constant force, and the restoration signal stabilization is also delayed after the force sensor 100 is released. A quick response force sensor needs to be developed for accelerating the response speed to accurately measure a correct force signal.
FIG. 2 shows a graph of Impedance vs Time for the prior art.
FIG. 2 shows an impedance I0 for a baseline signal level at time t0 for the force sensor 100 when the force sensor 100 is in standby. The force sensor 100 is depressed at time t1 with a constant force, and the impedance goes down towards Impedance I2 for a steady-state signal level. The actual signal goes down to I1 at time t1+3 s where impedance I1 is within 10% of impedance I2.
The 10% difference is calculated as follows:ABS(I1−I2)/I2≤10%; wherein
ABS: absolute value.
I1: Impedance for actual signal.
I2: Impedance for the steady-state signal level for a fixed force.
The force sensor 100 is released at time t2, the impedance goes up approaches the baseline signal level I0. The actual impedance signal reaches I3 at time t2+3 s where impedance I3 is within 10% of impedance I0 for the baseline signal level.
The 10% difference is calculated as follows:ABS(I0−I3)/I0≤10%; wherein
I3: Impedance for actual signal.
I0: Impedance for the baseline signal level.
It shows a force signal delay at least 3 second before the force signal being stable.
FIGS. 3A-3B show a peeling test for the prior art.
FIG. 3A shows the prior art before the peeling test.
FIG. 3A is the same as FIG. 1A which shows the force sensor 100 before peeling test being applied. A force sensor 100 is secured from bottom side and an upward peeling force is applied on the top substrate 11.
FIG. 3B shows the prior art after the peeling test.
FIG. 3B shows the PSA spacer 14 is torn apart and residues 141 of the PSA spacer 14 remains on a bottom surface of the top substrate 11 since the PSA spacer 14 is viscoelastic which has a cohesive force smaller than an adhesive force.